Absorbent system and method for capturing CO2  from a gas stream

ABSTRACT

The present invention relates to an absorbent, an absorbent system and a process for removing acidic gas such as CO 2  from exhaust gases from fossil fuel fired power stations, from natural gas streams, from blast furnace oven off-gases in iron/steel plants, from cement plant exhaust gas and from reformer gases containing CO 2  in mixtures with H 2 S and COS. The liquid absorbent, a mixture of amine and amino acid salt is contacted with a CO 2  containing gas in an absorber and CO 2  in the gas stream is absorbed into the liquid. The absorbed CO 2  forms more than one type of solid precipitate in the liquid at different absorption stages. In a first absorption stage solid precipitate of amine bicarbonate is formed and is withdrawn as slurry from the bottom of a first absorber section. In a second absorption stage solid precipitate of alkali metal bicarbonate is formed and withdrawn as slurry at the bottom of a second absorber section. The slurry withdrawn from the first absorption section is heated to dissolve the precipitate with CO 2  release in an amine flash regeneration tank. The slurry from the second precipitation stage is withdrawn from the bottom of the second absorber section and sent to a regenerator for desorption with CO 2  release. The lean amine and amino acid salt mixture from the flash regenerator and desorber are mixed and returned to the top of the absorber. This absorbent system improves carbon dioxide removal efficiency due to its higher CO 2  removal ability per cycle when compared with conventional amine, absorbent from organic acid neutralized with inorganic base and carbonate based absorbent system. It exhibits less solvent vaporization loss because part of the absorbent is in salt form.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a liquid absorbent and a system for capturingacidic gases, such as carbon dioxide from an exhaust gas. The inventionalso relates to a method of capturing acid gases from an exhaust gas ina process plant. The exhaust gas may come from fossil fuel fired powerstations, from natural gas streams, from blast furnace oven off-gases iniron/steel plants, from cement and fertilizer plant exhaust gas and fromreformer gases containing CO₂ in mixtures with H₂S and COS.

BACKGROUND OF INVENTION

The world is currently facing the challenge of global warming due tohigh levels of greenhouse gases particularly carbon dioxide emitted tothe atmosphere. The key challenge is on reducing CO₂ emissions at lowcost. Several technologies are being investigated and applied for thecapture and separation of carbon dioxide from flue gas streams includingsolvent, chemical looping, oxy-combustion, sorbent and membrane. Each ofthese technologies has areas best suited for application.

The method to remove acid gases from a gas stream using solvent is awell-known art. Typical absorbent solutions such as amine, carbonate,ammonia or amino acid salt solutions are used to remove the acid gas. Asimplified conventional layout of such an absorption plant includes theuse of an absorber and a desorber where the solution is circulated in acontinuous cycle where the absorbent liquid is contacted countercurrentwith the upward flowing gas. A main issue with these processes,especially in cases of removal of CO₂ from low partial pressure fluegases, is the energy required for regenerating the absorbent in thedesorber. Several technologies using the amine-based process have beendeveloped including the conventional technology like the Fluor Econamineprocess which has been further improved to the Fluor Econamine FC Plus℠process. Other amine base processes include MHI/KEPCO's KS-1, 2, and 3(sterically hindered amines); Cansolv® Absorbent DC101™ (tertiary amineswith a promoter); HTC Purenergy's mixed amine solvent and IFP's DMX.Promoted and un-promoted carbonate based process such as the Benfieldprocess has also been developed and used mostly in the areas with mediumto high pressures. An amino acid salt based process, The Alkazid processhas earlier been developed developed by I.G. Farbenindustrie to removeH₂S or mixtures of H₂S and CO₂ and more recently, amino acid salttechnology has also been developed by Siemens. A common challenge in allthe existing processes, in particular the low pressure processes, is thehigh regeneration energy requirement. For absorption technology to be aviable alternative for CO₂ removal from exhaust gas stream, remarkablereductions in energy requirement for such technology in particular forCO₂ removal from post combustion exhaust gases are needed.

Several patents and patent applications related to the technology whereamine, carbonate or amino acid based absorbents are used for acidic gasremoval have been disclosed.

U.S. Pat. No. 1,990,217 teaches the use of a salt of an organic acid andan inorganic base for gas purification, while U.S. Pat. No. 2,176,441teaches the use of a salt of amino acids and inorganic or organic baseswhereby the amino acid should be derived from a primary, secondary ortertiary amine having at least two nitrogen atoms. US 2007/0264180 A1discloses use of an absorbent solution comprising compounds capable offorming two separable phases by addition of an acid that is strongerthan the acid compounds of the gaseous effluent to be treated: a firstphase rich in acid compounds and a second phase poor in acid compoundswherein amine, amino acids or amino-acid alkaline salts are used asactivators to favour absorption of compounds to be eliminated. Theprocess includes acid neutralization of multiamines to form two phaseliquid and a process for separation of the two phase liquid.AU-B-67247/81 describes the use of an aqueous scrubbing solutioncomprising a mixture of a basic salt, potassium carbonate, and anactivator for the said basic salt comprising at least one, stericallyhindered amine and a sterically hindered amino acid as a cosolvent forthe sterically hindered amine. The amino acid here serves to prevent twophase separation of the aqueous solution at high temperatures.

BE 767,105 discloses a process for removing acid gases from gaseousstreams by contacting the gaseous streams with a solution comprisingpotassium carbonate and an amino acid.

These above-described prior processes involve improvements of thetraditional absorption/desorption cycle using liquid absorbentsthroughout the process.

WO 2012/030630 A1 teaches the use of a system comprising an amine and/oramino acid salt capable of absorbing the CO₂ and/or SO₂ to produce aCO₂- and/or SO₂-containing solution; an amine regenerator to regeneratethe amine and/or amino acid salt; and, when the system captures CO₂, analkali metal carbonate regenerator comprising an ammonium catalystcapable of catalyzing the aqueous alkali metal bicarbonate into thealkali metal carbonate and CO₂ gas. This disclosure does not allowprecipitation in the absorber. Precipitation is only allowed outside theabsorber in the amine regenerator unit where a concentrated alkalicarbonate is used to regenerate the amine/amino acid salts whileproducing alkali bicarbonate precipitate.

Solid precipitation in the absorber is typically avoided in mostprocesses for CO₂ removal, mainly because of potential pluggingproblems. WO 03/095071 A1 discloses a concept for the use of slurriesfrom precipitating amino acid salt for CO₂ capture. U.S. 2010/0092359 A1discloses a method for capturing CO₂ from exhaust gas in an absorber,wherein the CO₂ containing gas is passed through an aqueous absorbentslurry. The aqueous absorbent slurry comprises an inorganic alkalicarbonate, bicarbonate and at least one of an absorption promoter and acatalyst. CO₂ is converted to solids by precipitation in the absorber.Said slurry having the precipitated solids is conveyed to a separatingdevice, in which the solids are separated off, essentially all of atleast one of the absorption promoter and catalyst is recycled togetherwith the remaining aqueous phase to the absorber.

WO 2013/053853 A1 discloses a method for regeneration of bicarbonateslurry formed in a carbonate process.

The object of the present invention is to provide an absorbent systemand method that improves carbon dioxide removal efficiency when comparedwith conventional amine, absorbent from organic acid neutralized withinorganic base carbonate based and ammonia absorbent systems.

SHORT DESCRIPTION OF THE INVENTION

The present invention provides an absorbent for capturing CO₂ from anexhaust gas comprising an aqueous absorbing mixture of at least an amineand a fast reacting amino acid salt, wherein the amine is selected fromhigh bicarbonate forming amines, the mixture forms precipitates duringCO₂ absorption in the absorber.

Preferably, the high bicarbonate forming amines are sterically hinderedand/or are tertiary amines. Examples of preferred amines are:2-amino-2-methylpropanol (AMP), 2-amino-2-methyl-1,3-propanediol (AMPD),2-(diethylamino)-ethanol(DEEA), N,N-dimethylethanolamine (DMMEA) andmethyl diethanolamine (MDEA), triethanolamine (TEA),1-(diethylamino)-2-propanol, 3-(diethylamino)-1-propanol,tripropylamine, 2-pyrrolidinoethanol, 3-(diethylamino)-1,2-propanediol,N-piperidineethanol, 1-methylpiperidine-2-ethanol, 1-piperidinepropanol.

Amino acid salts are products of neutralization between an amino acidand an inorganic base or an organic base. In the present invention, theamino acid preferably has a pKa≥9.

Examples of suitable amino acids are glycine, taurine, sarcosine,proline, alanine, lysine, serine, pipecolinic acid, arginine, threonineand cysteine.

Examples of suitable inorganic bases are potassium hydroxide (KOH),sodium hydroxide (NaOH), lithium hydroxide (LiOH).

Examples of organic bases are amines selected from amines with pKa≥9,said amines include methylaminopropylamine (MAPA), piperazine (PZ),N-2-hydroxyethylpiperazine, N-(hydroxypropyl)piperazine, diethanoltriamine (DETA), 2-((2-aminoethyl)amino)ethanol (AEEA), piperidine,pyrrolidine, dibutylamine, trimethyleneimine, 1,2-diaminopropane and1,3-diaminopropane, N-2-hydroxyethylpiperazine,N-(hydroxypropyl)piperazine, monoethanolamone (MEA), diethanolamine(DEA), diisopropanolamine (DIPA), 3-aminopropanol (AP),2,2-dimethyl-1,3-propanediamine (DMPDA),3-amino-1-cyclohexylaminopropane (ACHP), diglycolamine (DGA),1-amino-2-propanol (MIPA), 2-methyl-methanolamine (MMEA) and piperidine(PE).

The aqueous absorbent mixture comprises from 0.5 to 8.0 mol/kg of thefast reacting amino acid salt, and preferably from 1 to 4 mol/kg. Theaqueous absorbent mixture comprises from 0.5 to 10.0 mol/kg of theamine, preferably from of 1.0-6.0 mol/kg.

The aqueous absorbent mixture comprises from 10 to 85% water.

Another aspect of the invention is a system for capturing CO₂ from anexhaust gas comprising:

-   (a) an absorber having at least two sections, or alternatively two    absorbers in series, containing the absorbent according to the    invention, wherein a first precipitate of amine bicarbonates is    formed in a first section and optionally a second precipitate of    metal bicarbonates is formed in a second section;-   (b) a flash regenerator wherein the first precipitate of amine    bicarbonates is dissolved, and CO₂ is released,-   (c) a desorber wherein the aqueous absorbent mixture of the amine    and the fast reacting amino acid salt is regenerated, and CO₂ is    released, and-   (d) a reboiler for supplying heat to the desorber.

Precipitation occurs in two steps, the first precipitation of aminebicarbonate complex occurs at an earlier stage in CO₂ absorption and thefirst precipitate is withdrawn at the end of the first absorber sectionand conveyed through a cross heat exchanger to a flash regenerator wherethe precipitate dissolves with CO₂ release. A low grade heat can be usedin the flash regenerator. A second precipitation of metal bicarbonatecomplex occurs at the end of the second absorber section or a CO₂ richabsorbent system at the end of the second section is conveyed through asecond cross heat exchanger to the absorbent regeneration unit/desorber.The heat of regeneration is supplied through a reboiler and the hot leanabsorbent is used as pre-heater for the CO₂ rich slurries withdrawn fromthe first and second sections of the absorber. The lean absorbents fromthe flash regenerator and the desorber are remixed and returned to thetop of the absorber to continue the cycle wherein the CO₂ released fromthe flash regenerator and the desorber is collected and sent tocompression. When there are two absorbers, a slurry of amine bicarbonatecomplexes from the first absorber is passed through a solid-liquidseparator from which the more concentrated slurry is conveyed to theflash generator where the precipitate dissolves with CO₂ release. Theliquid phase is returned to the top of the second absorber in acountercurrent manner with the upcoming gas stream to continue CO₂absorption.

When the system contains two absorbers, it further comprises asolid-liquid separator from which the more concentrated slurry isconveyed to the flash regenerator and from which the liquid phase isreturned to the top of the second absorber section in a countercurrentmanner with the upcoming gas stream to continue CO₂ absorption.

The inventive absorbent system improves carbon dioxide removalefficiency due to its higher CO₂ removal ability per cycle when comparedwith conventional amine, absorbent from organic acid neutralized withinorganic base and carbonate based absorbent system. It exhibits lesssolvent vaporisation loss because part of the absorbent is in salt form.

The present invention also provides a method of capturing CO₂ from anexhaust gas in a process plant comprising:

-   passing the exhaust gas through an absorber containing the absorbent    of the invention,-   enhancing precipitation of a first precipitate of amine bicarbonates    in a first section of the absorber forming a first rich CO₂    absorbent slurry,-   withdrawing the rich absorbent slurry from the first section to a    flash regenerator, in which the first precipitate of amine    bicarbonates is dissolved, and CO₂ is released,-   obtaining a rich CO₂ liquid absorbent in a second section of the    absorber,-   conveying the rich CO₂ liquid absorbent from the second section to a    desorber,-   wherein the rich CO₂ liquid absorbent is heated resulting in release    of CO₂ and a hot lean absorbent to be recycled to the absorber.

The slurry withdrawn from the first absorption section is heated todissolve the precipitate with CO₂ release in the amine flashregeneration tank. The first precipitate may be heated by heat transferwith the hot lean absorbent coming from the desorber. Alternatively, thefirst precipitate may be regenerated in the flash regenerator by anavailable low grade heat in the process plant.

A second precipitate of metal bicarbonates may occur in the secondsection of the absorber and form a second rich CO₂ absorbent slurry. Theslurry or the rich CO₂ liquid absorbent from the second section iswithdrawn from the bottom of the absorber and sent to the regeneratorfor desorption with CO₂ release. The slurry or the rich CO₂ liquidabsorbent from the second section may be passed through a second heatexchanger to be heated by heat transfer with the hot lean absorbent fromthe desorber.

The lean amine and amino acid salt mixture from the flash regeneratorand desorber are preferably mixed and returned to the top of theabsorber.

When two absorbers are used, slurry of the first precipitate from thefirst absorber section is passed through a solid-liquid separator fromwhich the more concentrated slurry is conveyed to the flash regenerator.The first precipitate dissolves with CO₂ release in the flashregenerator, and the liquid phase will be returned to the top of thesecond absorber section in countercurrent manner with the upcoming gasstream to continue CO₂ absorption.

FIGURES

FIG. 1 is a simplified sketch of the slurry-slurry process for CO₂capture.

FIG. 2 is a simplified sketch of the slurry-slurry process for CO₂capture with two separate absorber sections.

FIG. 3 shows XRD analysis result.

FIG. 4 shows an example vapour liquid solid equilibrium (VLSE) at 40° C.and 120° C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention came out of the desire of the inventors to shortenthe time required for the formation of solid precipitate in certainamino acid salt systems and increase the absorption rate of such system.Precipitation of CO₂ as solid in a process will increase the CO₂ drivingforce into the liquid and result in increased loading capacity. Certainamino acid salt systems such as potassium sarcosinate have very goodabsorption kinetics, (Aronu U E, Ciftja A F, Kim I, Hartono A ;Understanding precipitation in amino acid salt systems at processconditions, Energy procedia 37 (2013) 233-240), but formation of solidpotassium bicarbonate (KHCO₃) occurs very late at the point when theabsorption kinetic is very slow making it unfavorable precipitatingsystem candidate since the benefit of precipitation at process conditioncannot be fully exploited. The inventors therefore searched for a methodto enhance early precipitation for such a system. It is known thatcertain amines, such as 2-amino-2-methylpropanol (AMP), at high CO₂loading or high concentration can precipitate in a process. An absorbentthat can make the fast reacting amino acid salt to form an earlyprecipitate will be ideal. Various blends of this AMP and differentamino acid salt solutions were therefore prepared and CO₂ absorptionbehavior monitored in rapid screening set up with a possibility tomonitor the precipitation behavior at process conditions. It was alsofound that in some cases the mixture of the amino acid salt and AMPforms a complete homogeneous mixture, while in other cases the solutionseparates into two phases before CO₂ absorption. The solution formingtwo phases gradually reverts to a single phase as CO₂ absorptionprogresses before precipitation sets in. As CO₂ absorption proceeded,and it was found that solid precipitation occurred early when thereaction rate is still very high, and it was found that the morphologyof the precipitate is not the same as that of the metal bicarbonate,KHCO₃. The morphology which was crosschecked by using X-Ray Diffraction(XRD) analysis, conforms to that of the bicarbonate of the amine, AMP.Further, as the experiment continued at a later stage a new type ofcrystals forming in the mist of the first crystals was found. Thesesecond crystals were found to be KHCO₃ by using XRD. The firstprecipitate requires low temperature dissolution and amine regenerationwith CO₂ release, while a higher temperature dissolution and absorbentregeneration is required in the desorber for the second stageprecipitate. A test of the dissolution temperature of the precipitatesshows that the first precipitate dissolves from 50° C. and completelywith CO₂ release at 75° C., while the second precipitate dissolves from70° C. to 100° C. An example of an overall initial reaction is describedin Eq. 1:RNHCOO⁻M⁺+R′NH₂+2CO₂⇔⁻OOCRNHCOO⁻M⁺+R′NHCOO  (1)The overall stepwise reaction in such a system can be described asfollows:

-   Step 1: Formation of Solid Precipitate of Amine Bicarbonate    R′NH₂+CO₂⇔R′NHCOO  (2)    R′NHCOO+H₂O⇔R′NH₂+HCO₃ ⁻↓  (3)-   Step 2: Formation of Solid Precipitate of Metal Bicarbonate    RNHCOO⁻M⁺+CO₂⇔⁻OOCRNHCOO⁻M⁺  (4)    ⁻OOCRNHCOO⁻M⁺+H₂O⇔⁻OOCRNH₂+MHCO₃↓  (5)    R and R′ represent hydrogen, C₁₋₄alkyl, C₁₋₄alkanol, or a straight    chain, cyclic or aromatic amine groups, wherein at least one of R    and R′ is an C₁₋₄alkyl, C₁₋₄ alkanol or a straight chain, cyclic or    aromatic amine group, wherein the straight chain contains up to 7    carbon atoms, and the cyclic or aromatic amine groups contain from 3    to 6 carbon atoms. M can be selected from K, Li or Na

The amino acid salt used in the present invention is the product ofneutralization between an amino acid and an inorganic base or organicbase. The amino acids that can be used include but are not limited toglycine, taurine, sarcosine, proline, alanine, lysine, serine,pipecolinic acid, arginine, threonine and cysteine.

The inorganic bases that can be used for amino acid neutralization inthe present invention include but are not limited to potassiumhydroxide, sodium hydroxide and lithium hydroxide.

The organic bases that can be used for amino acid neutralization includeamines; such amines include but are not limited to:methylaminopropylamine (MAPA), piperazine (PZ),N-2-hydroxyethylpiperazine, N-(hydroxypropyl)-piperazine, diethanoltriamine (DETA), 2-((2-aminoethyl)amino)ethanol (AEEA), piperidine,pyrrolidine, dibutylamine, trimethyleneimine, 1,2-diaminopropane,1,3-diaminopropane, 2-amino-2-methylpropanol (AMP),2-(diethylamino)-ethanol(DEEA), 3-amino-1-cyclohexylaminopropane (ACHP),3-aminopropanol (AP), 2,2-dimethyl-1,3-propanediamine (DMPDA),1-amino-2-propanol (MIPA), 2-methyl-methanolamine (MMEA), piperidine(PE), monoethanolamone (MEA), diethanolamine (DEA), diisopropanolamine(DIPA).

Where an organic base such as amine is used for amino acidneutralization, it is preferred that the pKa of the amine is at leastgreater than the pKa of amino acid used.

The amine blended with the amino acid salt is preferably a strong orhigh bicarbonate forming amine like a sterically hindered and/or atertiary amine. Such amines include but are not limited to2-amino-2-methylpropanol (AMP), 2-amino-2-methyl-1,3-propanediol (AMPD),2-(diethylamino)-ethanol(DEEA), N,N-dimethyl-ethanolamine (DMMEA),methyl diethanolamine (MDEA), triethanolamine (TEA),1-(diethylamino)-2-propanol, 3-(diethylamino)-1-propanol,tripropylamine, 2-pyrrolidino-ethanol, 3-(diethylamino)-1,2-propanediol, N-piperidineethanol, 1-methylpiperidine-2-ethanol and1-piperidinepropanol.

In FIG. 1, a simplified process diagram of the method used to captureCO₂ using the absorbent according to the invention is disclosed. TheCO₂-containing gas stream 1 enters the absorber A1 in the bottom andflows upwards. It meets a liquid absorbent stream 12 in two stages. Atthe first contact stage in section 2, at the bottom of the absorber A1,stream 12 is a stream containing a slurry of water, a mixture of themetal bicarbonate of amine (Li, Na, or K); carbamates of amine, aminoacid; amino acid salt and amine. At the second contact stage in section1 of A1, at the point of withdrawal of stream 3, stream 12 is streamcontaining a slurry of water, a mixture of the bicarbonate of amine;carbamate of amine, amino acid; amino acid salt and amine. This impliesthat at each point of contact the aqueous phase is partially or fullysaturated with the bicarbonates such that the flow contains both solidsand liquid. In addition to the bicarbonates, the aqueous solutioncontains a precipitation promoting amine and an absorption ratepromoting amino acid salt or a precipitation promoting amino acid saltand an absorption rate promoting amine.

In FIG. 2, where the two sections of absorber A1 are in two differentcolumns, CO₂-containing gas stream 1 b from absorber section 2 entersthe bottom of absorber section 1 and flows upwards in contact with thedownward flowing stream 12. The withdrawn slurry, stream 3 is passedthrough a solid-liquid separator where more concentrated slurry, stream4 is sent to the flash regenerator, while the liquid phase, stream 3 bis returned to the absorber section 2 for contacting the upcomingCO₂-containing gas stream 1.

The operating temperature of the absorber will depend on the inlet fluegas temperature and will typically be from 30° C. to 80° C., preferablyfrom 40° C. to 60° C. Further cooling or pre-treatment of the flue gasmay be required to remove/reduce fly ash in cases with high temperatureand water content, some cooling and water removal might be necessary.Some cooling may be required in the bottom region of section 1 beforewithdrawal of stream 3 to further enhance solid precipitation in thisregion. The stream 3 region in the absorber is preferably maintained at30° C. to 50° C. In the absorber, the CO₂ is absorbed into the aqueousslurry and the exhaust, stream 2 with reduced CO₂ content leaves theabsorber, after a water wash section. This water wash is only needed toretain the amine, depending on its volatility. The absorption tower ispreferably a plate tower that can handle slurries. A spray tower, packedtower or any other suitable tower able to handle slurries can also beused. In the aqueous phase, the additional reactions described in Eq. 2and 3 take place in section 1 before the withdrawal of stream 3 whilethe additional reactions described in Eq. 4 and 5 takes place in section2 before the withdrawal of stream 7 at the bottom of the absorber.

The entering stream 12 will typically be high in amino acid salt andamine content. During contact with CO₂, the fast reacting amino acidsalt enhances the transport of gaseous CO₂ into the liquid absorbent.The CO₂ in the liquid phase is further stored away into the amine asbicarbonate. As the gas liquid contact continues down the absorber,amine bicarbonate formed grows until the solubility limit is exceededresulting in precipitation as solid amine bicarbonate slurry in thebottom of section 1, where the bicarbonate slurry is withdrawn as stream3 at a temperature from 30° C. to 60° C. Withdrawal of stream 3 has asignificant benefit; complete withdrawal of CO₂ saturated slurry fromthe absorber will shift equilibrium further to the right enhancing moreCO₂ containing liquid phase products. The slurry withdrawal will alsosignificantly reduce the viscosity of the absorbent system at thisstage. This combined effect will accelerate mass transport of CO₂ intothe liquid phase even at the middle of the absorption resulting inenhanced absorption capacity. Further, a withdrawal of the firstprecipitate from the solution as stream 3 will result in a less volumeof solution for regeneration in the desorber. The amino acid saltcontaining CO₂ in low proportion and amine saturated with CO₂ ascarbamate and/bicarbonate will remain in solution and continuecontacting CO₂ in the absorber section 2. Ability of the amino acid saltfor fast reaction with CO₂ and increased gas to liquid in this sectionallows further uptake of CO₂ along the absorber in section 2 formingmore amino acid carbamate, which undergoes hydrolysis to produce KHCO₃as in Eq. 5. At the solubility limit, KHCO₃ will precipitate and form aslurry containing amine bicarbonate/carbamate as well as amino acidcarbamate/amino acid. This second stage precipitation in absorbersection 2 will also result in another absorption rate and capacityenhancement since CO₂ bound as precipitate will not participate in theequilibrium backpressure over the solution. Depending on the flue gastemperature, the slurry leaves the bottom of the absorber as stream 7 ata temperature from 40 to 70° C. The slurry in stream 7 is passed throughthe cross exchanger HX1 and is heated up like in a conventional amineprocess by heat transfer with the lean absorbent stream 9 from thedesorber. According to the invention, the filtration and/orcrystallization often proposed for a precipitating process is notrequired. The slurry is sufficiently saturated with solids and a smallerliquid volume will be treated in the desorber because part of the totalliquid has been withdrawn as stream 3 as precipitate slurry. The heatedslurry stream 8 is delivered into the desorber D1 from the top where thesolid precipitate is completely dissolved with CO₂ released in stream13. CO₂ desorption is enhanced on further contact with upcomingstripping vapour from a reboiler R1. The precipitate in the slurry maybe completely dissolved before in enters the reboiler through stream 15depending on the desorber operating conditions. Hot vapour from thedesorber is returned into the bottom of the desorber column by stream16. The typical temperature range in the desorber is 100° C. to 200° C.In the desorber the KHCO₃ decomposes with CO₂ release likewise thebicarbonate/carbamate of the amine and amino acid resulting in theregeneration of the amino acid salt and amine in the absorbent.

The desorber can be a packed tower, a plate tower, a spray tower, aflash tank or any other suitable tower.

Stream 9 emerges the cross changer as stream 10 with lower temperature60° C. -110° C. but with sufficient heat to transfer at the crossexchanger HX2 to stream 3. Stream 3 emerges from HX2 as stream 4 athigher temperature 60-110° C., sufficient to completely dissolve theprecipitate and further strip off CO₂ from the liquid phase. The stream4 is fed into the flash regenerator F1 where CO₂ is released in stream 6and combined with stream 13 to form stream 14 containing CO₂ forstorage. In addition, it could be possible to use some waste/low heatstreams to heat up stream 3 and/or 4. CO₂ pressure for compression willvary significantly based on the mode of operation of the desorber. Theproduced CO₂ pressure can be in the range 3-50 bar. Stream 10 emergesfrom HX2 as stream 11 at lower temperature and is combined with stream5, lean absorbent from the flash regenerator, F1 to form stream 12; thelean absorbent is returned to the top of the absorber A1. Stream 5 canalso be combined with stream 8 delivered into the desorber D1. Theprocess can also be configured such that the lean stream 9 is first usedto heat up stream 3 in HX2 before using it to heat up stream 7 in HX1.

EXAMPLE 1

Absorption test experiments were carried out on a 7 mol/kg solutioncontaining 3 mol/kg potassium sarcosine (KSAR) and 4 mol/kg2-amino-2-methylpropanol (AMP) charged in a jacket glass reactor. Thereactor is equipped with Particle Vision and Measurement (PVM) andFocused Beam Reflectance Measurement (FBRM) probes for monitoringprecipitation. Absorption at 40° C. starts after calibration of CO₂analyser with CO₂-N₂ gas mixture containing 10 vol % CO₂ with flow rate2.5 NL.min⁻¹. Same gas mixture is then bubbled through a 375 mol/kg ofthe absorbing solution while the solution is agitated using a stirrer at300 rpm. The gas phase leaving the reactor is cooled and CO₂ content isanalysed online by IR. The absorption test gives fast relativecomparison of absorption rate, it also allows the possibility to studythe precipitate behaviour, crystal formation and dissolution during theexperiment. The absorption process terminates when the concentration ofCO₂ in the effluent reaches 9.5 vol % representing about 9.5 kPa partialpressure of CO₂ or when the absorption rate becomes too low. A liquidsample containing both the rich liquid and precipitated crystal iscollected for analysis at the end of absorption. In addition, a similarsample is collected, filtered and dried for precipitate analysis by XRD.Precipitate dissolution was monitored by heating up the solvent in therange 40° C. to 90° C. while bubbling pure N₂ gas at 2.25 NL/min throughthe solution in the reactor bottle. It was observed that the CO₂ contentof the effluent gas increases as N₂ bubbles through the solution whilethe precipitate dissolution is monitored and logged. Gas phase analysiswas used to determine the liquid phase CO₂ concentration during theexperiment.

EXAMPLES 2-3

An absorption experiment was carried out in the same manner as inExample 1, except that a 7 mol/kg solution containing 3mol/kg potassiumglycine (KGLY) and 4 mol/kg 2-amino-2-methylpropanol (AMP) as anabsorbent was used as an absorbing solution in Example 2. This absorbentwas found to form two liquid phase before CO₂ absorption, but forms onephase as the loading progresses before precipitation start. In example3, a 7 mol/kg solution containing 3 mol/kg sodium glycine (NaGLY) and 4mol/kg 2-amino-2-methylpropanol (AMP) was the absorbent used. Theresults obtained are shown in Table 1.

TABLE 1 Amino 1st Precipitation 2nd Precipitation Crystal acid saltAmine Initial abs rate Absorption rate Loading Absorption rate LoadingLoading Dissolution [mol/kg] [mol/kg] [mol/m3/min] [mol/m3/min][molCO2/kg] [mol/m3/min] [molCO2/kg] [molCO2/kg] Start (° C.) Example 13 m KSAR 4 m AMP 34.3 29.6 0.81 6.5 2.54 3.06 55 Example 2 3 m KGLY 4 mAMP 25.5 19.0 0.99 9.2 2.62 3.34 50 Example 3 3 m NaGLY 4 m AMP 23.023.2 0.30 7.7 2.07 2.39 65 Comparative 5 m KSAR 36.0 3.9 2.89 3.23Example 1 Comparative 4 m AMP 13.4 3.9 1.91 2.39 Example 2 Comparative4.9 m MEA 24.3 2.68 Example 3

COMPARATIVE EXAMPLES 1-3

An absorption experiment was carried out in the same manner as inExample 1, except that an aqueous solution containing 5 mol/kg potassiumsarcosine (KSAR) as an absorbent in comparative example 1 while incomparative example 2-3 aqueous solution containing 4 mol/kg2-amino-2-methylpropanol (AMP) and 4.9 mol/kg of monoethanolamine (MEA)was used, respectively.

From the result in Table 1, it can be seen that the type of absorbentused in Examples 1-3 forms more than one precipitate before absorptionis terminated at 9.5 vol % CO₂ out of the reactor. Here precipitationoccurred in two steps. In comparative example 1-2, it can be observedthat only one precipitate is formed. In addition, it can be observedwhen looking at the absorption rate that the first precipitate inExample 1-3 occurs when the absorption rate is still high and theloading is low, and the second precipitate occurs when the absorptionrate is low and loading is high. In the comparative example 1-2, theonly precipitate occurs when the absorption rate is low and the loadingis high. In the comparative example 3, no precipitate is formed.

EXAMPLE XRD

The slurry formed after absorption experiment was filtered and thefiltrate dried at room temperature. X-ray diffraction (XRD) analysis wasconducted on the filtered solid cake. The XRD analysis results are shownin FIG. 3. The filtrate from the comparative example 1 was identified asKHCO₃ by XRD analysis, while the filtrate from comparative example 2 isidentified as the bicarbonate of AMP. From the figure, it can beobserved that the solid phase spectra of Example 1 and 2 contain spectraof comparative example 1 and 2 at variable degrees. This shows that twoprecipitate types are found in the solution and these two precipitatesare a mixture containing the precipitate of comparative example 1 and 2.During visual observation and monitoring using PVM, the experiment showsthat the first precipitate in Example 1 and 2 is needle-like as wasobserved in comparative example 2. The spectra confirm that formation offirst precipitate of AMP bicarbonate and as loading further increases asecond precipitate, KHCO₃ is formed. The absorption solid phase ofExample 3 appears somewhat different from the rest of the spectra.Although two precipitation stages occurred, precipitates with differentmorphology are formed. Example 3 is the only solution containing sodiumsalt; the other precipitates contain potassium salt.

EXAMPLE VLSE

An example vapour liquid solid equilibrium (VLSE) at 40 and 120° C. isshown in FIG. 4 for a precipitating absorbent according to theinvention. The figure shows that precipitate formation enables increasein the driving force of CO₂ into solution with the formation of a ‘flatplateau region’ at 40° C. where the increase in CO₂ loading does notresult in a corresponding increase in CO₂ partial pressure. This enableshigher CO₂ loading in this precipitating system when compared to systemsthat do not precipitate.

The invention claimed is:
 1. A system for capturing CO₂ from an exhaustgas comprising: (a) an absorber having at least two sections containingan absorbent, wherein the absorbent consists of an aqueous absorbingmixture consisting of 0.5 to 10.0 mol/kg of 2-amino-2-methylpropanol,0.5 to 8.0 mol/kg an amino acid salt of sarcosine or glycine, and water,wherein a first section of the absorber is configured to form a firstprecipitate of amine bicarbonates, wherein a second section of theabsorber is configured to form a second precipitate of metalbicarbonates, wherein the first section comprises an outlet configuredto remove a slurry comprising the first precipitate of aminebicarbonates from the first section, and wherein the second sectioncomprises an outlet configured to remove a slurry comprising the secondprecipitate of metal bicarbonates from the second section, (b) a flashregenerator configured to dissolve the first precipitate of aminebicarbonates and release CO₂, (c) a desorber configured to regeneratethe aqueous absorbing mixture and release CO₂, and (d) a reboilerconfigured to supply heat to the desorber.
 2. The system according toclaim 1, wherein the two sections are two sections within one absorberor are two separate absorbers in series.
 3. The system according toclaim 1, wherein the outlet in the first section is configured to removethe slurry of the first precipitate of amine bicarbonates from the firstsection and supply the slurry of the first precipitate of aminebicarbonates to the flash generator.
 4. The system of according to claim1, further comprising a first heat exchanger (HX1) arranged between theabsorber and the desorber.
 5. The system of according to claim 1,further comprising a second heat exchanger (HX2) arranged between theabsorber and the flash regenerator.
 6. The system according to claim 1,wherein the absorbent consists of an aqueous absorbing mixtureconsisting of 1.0 to 6.0 mol/kg of 2-amino-2-methylpropanol, 1 to 4mol/kg an amino acid salt of sarcosine or glycine, and water.